The present invention relates to a process for obtaining high-strength, composite martensitic/austenitic iron-chromium-manganese-carbon steel alloys. These steels find extensive use in the production of plates, rounds, chains, and the like, in plates for the mining and agricultural industries, in ordnance and as pressure vessel steels in the nuclear and chemical process industries. The high strength of the alloys in combination with other attractive properties such as corrosion and oxidation resistance yields a steel which has excellent potential as a high technology material.
The desired microstructural condition of a particular steel depends very much on the intended end use of the steel. For example, in the fossil fuel industry, where temperatures on the order of 500.degree. C. are quite normal, resistance to creep, oxidation, corrosion and catastrohic intergranular embrittlement is necessary. Therefore, in such an application the steel is often used in the 650.degree. C. tempered condition. In contrast, in the mining industry and in military applications, e.g., armored plates, room temperature and lower temperature properties are of much greater concern, and thus, strength and toughness become more critical parameters for such a steel. In addition, improved toughness and hardness improve wear resistance which is important in mining and agriculture. In seeking to attain these desired properties, the problem is complicated when the alloy content of structural steels is increased, because there is a tendency towards lower toughness values, especially if the steel is untempered. There is thus a need to attain for certain applications a high-strength steel, while still using high-alloy content to improve corrosion resistance.
A high-strength, ternary iron-chromium-carbon steel is disclosed in J. McMahon and G. Thomas, Proc. Third Intern. Conf. on the Strength of Metals and Alloys, Cambridge, Inst., Metals, London, 1, p. 180 (1973). An iron/0.35 weight % carbon/4 weight % chromium alloy is disclosed exhibiting a Charpy-V-Notch value of 12-15 ft/lbs and a plane strain fracture toughness (K.sub.Ic) of about 70 KSI-in.sup.1/2.
A further improvement in steel alloys is disclosed in U.S. Pat. Nos. 4,170,497 and 4,170,499 wherein a third alloying element (which is an austenite stabilizer, such as nickel or manganese) is added to increase the toughness and the stability of the austenite films. These patents also describe heat treatment processes for grain refining. By contrast, according to the present invention an object is to effect grain refining of the composite structure by refining the grain and packet size without disturbing the essential features of the autotempered lath martensite surrounded by stable austenite films, while at the same time increasing the chromium content to up to 13% to effect improved corrosion resistance.